A description of the benefits of variable valve actuation is given in application Ser. No. 08/246,274. Optimal operation of valves requires suitable variation of lift, duration and phase. In that patent application both variable lift and duration are described with the mechanism that moves the pivot of a rocker arm.
Variation of phase has been accomplished in a number of different ways. One of the simplest methods to comprehend is to alter the phase between the crankshaft and the intake camshaft in a dual overhead camshaft layout. This may occur with a drive mechanism than incorporates splines on the camshaft and a driving drum, as shown in SAE paper 901727. During operation the driving drum and camshaft are moved relative to each other while the cam drive undergoes normal operation.
A more complicated means of achieving phase variation is to use a three dimensional cam that is moved axially. This type of system, as described in U.S. Pat. Nos. 3,618,574 by Miller and 5,080,055 by Komatsu, et al., can be used to vary lift, duration and phase in a variety of ways.
Another simple method to control phase is that of switching between two independent cam profiles for low and high speed operation. Examples of this approach are described in U.S. Pat. Nos. 2,934,0452 by Longenecker, 4,151,817 by Mueller, 4,205,634 by Tourtelot, 4,970,997 by Inoue, et al. and 5,113,813 by Rosa. In this case, phase may be altered in a step-wise fashion from one angle to a second angle. Such a mechanism also allows the possibility of a step variation of lift and duration.